This invention relates to novel MBS impact modifiers, especially for PVC, and a process for preparation of the modifier.
Prior MBS impact modifiers are known, but whereas high rubber levels were always desired, achievement of such levels usually required isolation by coagulation procedures. While the advantages of spray drying as an isolation technique are well known, no one previously used spray drying methods for high butadiene content MBS modifiers because of problems which would be associated with the spray drying process with such polymers. Even with coagulation technique, greater than 70% rubber content was not thought achieveable. See French Pat. Nos. 1,457,829; 1,391,111; U.K. Pat. Nos. 1,039,802; 1,344,891; and U.S. Pat Nos. 3,288,886; 3,775,514; 3,644,250; 3,644,249; 3,671,610. The latter discloses up to 75% rubber content, but does not exemplify such high rubber levels. It was unexpected that higher rubber levels than were achieved with coagulation techniques could be achieved by spray dry techniques.
It is an object of the present invention to produce high, especially greater than 70% by weight, rubber content MBS graft polymers as improved impact modifiers.
This object, and others as will become apparent from the following disclosure, are achieved by the present invention which comprises in one aspect a process for producing a multiple stage, sequentially produced, impact modifier having a high rubber content comprising graft polymerizing methyl methacrylate and styrene in any order or proportion onto a polymerized butadiene or butadiene-styrene rubber polymer so as to form a latex of a two or more stage graft polymer, spray drying the resultant graft polymer latex while blowing fumed silica or another inert particulate material into the gaseous spray drying medium in the spray dryer, the rubber content of the resultant graft polymer being at least about 70% by weight, and in another aspect the product produced by this process.
The MBS polymer is dried from a latex which is polymerized sequentially so as to produce a graft polymer, the first stage being a rubbery polymer of butadiene, either alone or with one or more ethylenically unsaturated monomers such as methacrylates, acrylate, or styrene, the latter being most preferred. Optionally, one or more polyunsaturated cross-linking monomers are present in addition to butadiene.
The first stage, or "rubber" stage, i.e. the butadiene-containing stage, is preferably at least about 70% and more preferably at least about 75% with the balance being at least one subsequent stage.
The preferred fumed silica is produced by vapor phase hydrolysis of silicon tetrachloride at high temperatures, usually around 1100.degree. C., and is an amorphous material having a small particle size (about 0.007 to 0.05 microns) and a large surface area (about 50 to 390 m.sup.2 -g.sup..sup.-1). Silicas of this nature are wettable but can be made hydrophobic by silanization. Both forms are useful in the process of the invention.
Possibly other inert particulate material than fumed silica could be useful in the invention, especially if the inert particulate material has a particle size of about 0.007 to 1.5 microns and a surface area of about 1 to 400 m.sup.2 -g.sup..sup.-1 ; for example, aluminum silicate, calcium carbonate, carbon black, titanium dioxide, polymethyl methacrylate, zinc oxide, and infusorial earth.